1. Field of the Invention
The present invention relates to a process for producing 3,3',4,4'-tetramethyldiphenylmethane. More particularly, it pertains to a process for efficiently producing 3,3',4,4'-tetramethyldiphenylmethane, the intermediary raw material for 3,4,3',4'-benzophenonetetracarboxylic dianhydride which is important as the starting material for the carboxylic acid component that is the raw material for polyimide resin and as that for heat resistant high molecular compounds such as the curing agent for epoxy resins.
2. Description of the Related Arts
It has heretofore been known that 3,3',4,4'-tetramethyldiphenylmethane (hereinafter sometimes referred to as "3,3',4,4'-TMDM") is useful as the starting material to be oxidized into 3,4,3',4'-benzophenonetetracarboxylic dianhydride (hereinafter sometimes referred to as "BTDA"), but nothing has been known regarding the process for efficiently producing 3,3',4,4'-TMDM in high yield. Thus, BTDA has been produced by the process wherein two molecules of o-xylene and one molecule of acetaldehyde are subjected to condensation reaction in the presence of a large amount of sulfuric-acid catalyst to provide 1,1-bis(3,4-dimethylphenyl)ethane, which is then oxidized by nitric acid to anhydrate itself into BTDA as the product.
The above-mentioned conventional process, however, has suffered the disadvantage that relatively large amounts of isomers and high boiling substance are produced as the by-products, which are not only unusable but also responsible for decrease in the yield of the objective product in the oxidation step.
Aside from the above, o-xylene and formaldehyde are reacted in the presence of such acid catalyst as sulfuric acid or p-toluenesulfonic acid to produce tetramethyldiphenylmethane (hereinafter sometimes referred to as "TMDM"), which however, contains three types of major isomers, namely 3,3',4,4'-TMDM; 2,3,3',4'-tetramethyldiphenylmethane (hereinafter sometimes referred to as "2,3,3',4'-TMDM"); and 2,2',3,3'-tetramethyldiphenylmethane (hereinafter sometimes referred to as "2,2'3,3'-TMDM").
Having a higher melting point as compared with the other isomers, the objective 3,3',4,4'-TMDM can be separated and recovered from TMDM containing the above-mentioned three types of isomers by means of crystallization (refer to Vopr. Khim. Tekhnol., 71,112-114 (1983), USSR).
However, the aforestated conventional process alone wherein the catalyst such as sulfuric acid or p-toluenesulfonic acid is employed to produce TMDM, from which 3,3',4,4'-TMDM is separated and recovered by means of crystallization is available only in low yield of the objective 3,3',4,4'-TMDM. It is, therefore, necessary to contrive the improvement in the yield of the objective 3,3',4,4'-TMDM by improving the selectivity of reaction and the like.